Oxygen transfer rate and the corresponding power requirement to operate the rotor are vital for design and scale-up of surface aerators. Present study develops simulation or scale-up criterion correlating the oxygen transfer coefficient and power number along with a parameter governing theoretical power per unit volume (X, which is defined as equal to F4/3R1/3, where F and R are impellers' Froude and Reynolds number, respectively). Based on such scale-up criteria, design considerations are developed to save energy requirements while designing square tank surface aerators. It has been demonstrated that energy can be saved substantially if the aeration tanks are run at relatively higher input powers. It is also demonstrated that smaller sized tanks are more energy conservative and economical when compared to big sized tanks, while aerating the same volume of water, and at the same time by maintaining a constant input power in all the tanks irrespective of their size. An example illustrating how energy can be reduced while designing different sized aerators is given. The results presented have a wide application in biotechnology and bioengineering areas with a particular emphasis on the design of appropriate surface aeration systems. Biotechnol. Bioeng. 2007;96: 464–470. © 2006 Wiley Periodicals, Inc.